A bushing is a lead-trough structure via which a conductor can pass. Bushings are commonly used in transformers and other high voltage equipment. To obtain a well defined electrical field distribution inside and along a bushing, aluminum foils are wrapped in between paper insulation on the inside of the bushing to capacitively control the electrical field such that electrical field stress can be reduced and breakdown is avoided.
When the aluminum foil has been wrapped one turn around the inside of the bushing, i.e. when the foil has been wrapped for a full 360°, one end of the foil is applied another 10 to 100 mm to form an overlap with the other end of the foil with a paper insulation layer in between. This is illustrated in FIG. 2, where three pieces of aluminum foil have been wrapped inside the bushing with paper insulation layers arranged between each piece of aluminum foil.
The aluminum foil overlap area in the bushing has been identified as a problem since it effectively forms an electric resonance circuit that is excited by very fast transients (VFTs) which are caused by factors such as e.g. switching operations, faults and disturbances. These transients cause high overvoltages between the foils and occasionally lead to breakdowns in the bushings. In oil-impregnated paper bushings, this problem has been solved by short-circuiting every aluminum foil overlap by punching through both the two foils and the intermediate paper insulation with a sharp tool. When producing resin-impregnated paper bushings, it has not been possible to adapt this technique. The foil overlap has been used because it has been a common opinion that the edges of the foil cannot be exposed in the strong electrical field occurring during operation of the bushing.